1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with a magnetic recording medium suitable for high density recording.
2. Description of the Prior Art
Ferromagnetic powders which have hitherto been used for magnetic recording media are maghemite, cobalt-doped maghemite, magnetite, cobalt-magnetite Berthollide compounds of maghemite and magnetite, cobalt-doped Berthollide compounds of maghemite and magnetite and chromium dioxide. However, these oxide-type ferromagnetic powders are not so suitable for magnetic recording of a signal of short recording wavelength (about 2 .mu.m or less) or for magnetic recording with a narrow track width (about 100 .mu.m or less) because their magnetic properties such as coercive force (Hc) and residual magnetic flux density (Br) are insufficient for high density recording.
Development of ferromagnetic powders having properties suitable for high density recording has lately been carried out vigorously and one of these material is a ferromagnetic metal powder.
The following methods are known as a method of preparing such a ferromagnetic metal powder:
(1) A method comprising heat-decomposing an organic acid salt of a ferromagnetic metal and reducing with a reducing gas, which is described in, for example, Japanese Patent Publication Nos. 11412/1961, 22230/1961, 14809/1963, 3807/1964, 8026/1965, 8027/1965, 15167/1965, 16899/1965, (U.S. Pat. No. 3,186,829), 12096/1966, 14818/1966 (U.S. Pat. No. 3,190,748), 24032/1967, 3221/1968, 22394/1968, 29268/1968, 4471/1969, 27942/1969, 38755/1971, 4286/1971, 38417/1972, 41158/1972, 29280/1973 and Japanese Patent Application (OPI) No. 38523/1972.
(2) A method comprising reducing an acicular iron oxyhydride, acicular iron oxyhydride containing another metal or acicular iron oxide derived from these oxyhydrides, which is described in, for example, Japanese Patent Publication Nos. 3862/1960, 11520/1962, 20335/1964, 20939/1964, 24833/1971, 29706/1971, 30477/1972 (U.S. Pat. No. 3,598,568), 39477/1972, 24952/1973, 7313/1974, Japanese Patent Application (OPI) Nos. 5057/1971 (U.S. Pat. No. 3,634,063), 7153/1971, 38525/1972, 79153/1973, 82395/1973, 97738/1974, U.S. Pat. Nos. 3,607,219, 3,607,220 and 3,702,270.
(3) A method comprising evaporating a ferromagnetic metal in a low pressure inert gas, which is described in, for example, Japanese Patent Publication Nos. 25620/1971, 4131/1972, 27718/1972, Japanese Patent Application (OPI) Nos. 25662-25665/1973, 31166/1973, 55400/1973, 81092/1973, Japanese Patent Publication Nos. 15320/1974 and 18160/1974.
(4) A method comprising heat-decomposing a metal carbonyl, which is described in, for example, Japanese Patent Publication Nos. 1004/1964, 3415/1965, 16868/1970, 26799/1974, U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882.
(5) A method comprising electrodepositing a ferromagnetic metal powder using a mercury cathode and then separating the metal powder from mercury, which is described in, for example, Japanese Patent Publication Nos. 12910/1960, 3860/1961, 5513/1961, 787/1964, 15525/1964, 8123/1965, 9605/1965 (U.S. Pat. No. 3,198,717), 19661/1970 (U.S. Pat. No. 3,156,650) and U.S. Pat. No. 3,262,812.
(6) A method comprising reducing a solution containing a ferromagnetic metal salt with a reducing agent, which is described in, for example, Japanese Patent Publication Nos. 20520/1963, 26555/1963, 20116/1968, 9369/1970, 14934/1970, 7820/1972, 16052/1972, 41718/1972, 41719/1972 (U.S. Pat. No. 3,607,218), Japanese Patent Application (OPI) Nos. 1353/1972 (U.S. Pat. No. 3,756,866), 1363/1972, 42252/1972, 42253/1972, 44194/1973, 79754/1973, 82396/1973, U.S. Pat. Nos. 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556, 3,663,318, 3,669,643, 3,672,867, 3,726,664, Japanese Patent Application Nos. 91498/1973, 92720/1973, 106901/1974 and 134467/1974.
In these methods for the production of ferromagnetic metal powders, the ferromagnetic metal powders obtained by reducing in a reducing gaseous stream as in the case of (1) or (2) have an acicular grain form and a grain size of 200 to 1000 A in short length axial ratio being within a range of 3 to 20, which are suitable for use as a magnetic recording medium. There are formed sometimes pores in the grains depending on the feature of the production method. Ferromagnetic metal powders are readily be obtained having a coercive force (which will hereinafter be referred to as "Hc") of 400 to 1500 Oe and a saturated magnetization (which will hereinafter be referred to as ".sigma.s") of 100 to 180 emu/g. In the above described methods (3), (4), (5) and (6), ferromagnetic metal powders are obtained in such a manner that globular or granular grains are arranged at a constant interval with some orientation or contacted with each other because a magnetic field is applied to the ferromagnetic powder during the production thereof, and sometimes these grains are orientated in an irregular state. In any case, these grains are chained like a necklace, which are called "chain grains", and those having magnetic properties such as Hc of 300 to 2000 Oe and .sigma.s of 70 to 140 emu/g are readily obtainable. As a grain size, a grain diameter of 150 to 800 A and grain length of 500 to 10000 A are desirable for magnetic recording media.
Magnetic recording media using the above described acicular grains alone have the following disadvantages:
(1) The squareness ratio (Br/Bm, hereinafter referred to as "SQ") is low and it is difficult to obtain a practical squareness ratio, i.e., 0.75 or more. Therefore, the self-demagnetization is large and a high sensitivity (5 MHz VS) cannot be obtained in spite of a large Bm (maximum magnetic flux density).
(2) The dispersibility with binders and the surface smoothness of a tape prepared using these grains are so bad that there are much noise and a low S/N (signal/noise) ratio.
(3) Head abrasiveness is large.
On the other hand, magnetic recording media using the above described chain grains alone have the following disadvantages:
(1) The saturated magnetization is smaller than when using the acicular grains.
(2) The apparent density is small and handling of the grains is hard. Therefore, a tape prepared using the chain grains should be subjected to supercalendering treatment because of its soft surface.
(3) The cost for the production thereof is higher than when using the acicular grains.
(4) The durability to contact with heads is shorter.
Various studies or efforts have hitherto been made on both the grains in order to solve the above described problems, but these problems have not been solved completely, which hinder practical use of these grains.
We, the inventors, have made efforts to provide a magnetic recording medium free from the above described disadvantages and consequently have reached the present invention, in particular, getting a hint from the prior art magnetic recording media having two or more different layers as magnetic layers.
Magnetic recording media having a multi-layer structure, for example, consisting of a .gamma.-Fe.sub.2 O.sub.3 layer coated on a Fe.sub.3 O.sub.4 powder layer are well known whereby the disadvantages of Fe.sub.3 O.sub.4, i.e., oxidation in air or poor reprinting property can be solved with keeping the high magnetic flux density of Fe.sub.3 O.sub.4. It is also known that the disadvantage of CrO.sub.2, i.e., head abrasiveness can be improved by the provision of a multilayer structure consisting of a CrO.sub.2 type magnetic layer and iron oxide magnetic layer coated thereon and that the sensitivity can be increased and the harmonic distortion factor and S/N property can be improved by the provision of a multilayer layer consisting of a plurality of layers differing in coercive force.
These known facts are disclosed in, for example, Utility Model Publication No. 18135/1959, Japanese Patent Publication Nos. 2218/1962, 8106/1964, 23678/1964, 5351/1965, 185/1968, 28681/1973, Japanese Patent Application (OPI) Nos. 18508/1972, 37903/1972, 31907/1973, 39995/1973, 81093/1973, 98803/1973, 55304/1974 and U.S. Pat. Nos. 2,643,130, 2,647,954, 2,691,072, 2,941,901, 3,052,567, 3,185,775, 3,328,195, 3,416,949, 3,676,217 and 3,761,311.